I have some limited experience with LED lighting, sir.
Try this place - it's online sales only (afaik) and I just got some
4410 equivalent festoon lights from them to replace the incandescent
courtesy lights in the door steps and the cab dome light. They aren't a
perfect fit for the Chevy bulb holders, the flanges are a bit off,
but they do fit, work, and look GREAT! And they use about 1/15 the power.
Somewhere on their site there's an automotive incandescent to LED
chart, so you can figure out what you've got, and what you need to
replace it with.
Any more questions, email them, they're really good at LED support stuff.
Or ask markopolo. He also has some experience with LEDs.